Three-pipe splits

To provide either cooling or heating at any of the fan-coil units in a system requires a supply of hot gas. This will normally come from a third pipe, taken from the compressor discharge with a feed through a valve to the top of the indoor coil, and a drain back to the main liquid line. A non-return valve is needed to prevent the hot gas flowing back to the compressor suction. Speed control of the condensor fan and if available, of the compressor is needed to keep the discharge pressure high enough to supply the required hot gas.


The use of a number of components connected in this way implies that they are integrated into a coherent circuit with compressors, fans, solenoid valves etc. under a common control system. A few major manufacturers in the world are capable of engineering a complex system of this sort and supplying matching components and training the staff to instal and maintain it.
The outdoor unit is connected to a distribution device local to a group of fan-coils, and the direction of gas and liquid flow will be determined by the overall balance of load, whether cooling or heating. If most indoor units call for cooling, the flow will be as follows.

Discharge gas from the compressor passes through a four-port valve into the outdoor coil, and is partly condensed to a high-pressure mixture of liquid and gas. The liquid is separated in the distributor unit, and passed through two stages of pressure reduction, evaporates in the indoor unit and is returned as a low-pressure gas, through a controlling solenoid valve to the compressor. If any room calls for heating, the solenoid valves change over – the ‘cooling’ solenoid closing and the ‘heating’ solenoid opening.

This admits hot gas from the top of the separator to the coil, where it gives up its heat to the room air, and condenses. This liquid flows into the liquid header in the distributor, and can then pass directly to a ‘cooling’ unit.

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